1. Field of the Invention
The present invention relates to the field of magnetic resonance imaging, and particularly to a body coil for a magnetic resonance imaging apparatus.
2. Description of the Prior Art
Magnetic resonance imaging (MRI) makes use of the phenomenon of nuclear magnetic resonance (NMR); according to different attenuation rates of released energy in different structural environments inside an object, and by way of an externally applied gradient magnetic field for detecting electromagnetic waves transmitted therefrom, it can acquire the knowledge of the positions and species of the atomic nuclei composing the object, and based on this it can draw images of the internal structure of the object.
MRI devices are widely used in examination and treatments in clinical medicine and an MRI device in the prior art mainly comprises a frame, a patient bed, a gradient coil, a radio frequency (RF) coil and a control and image reconstruction system. When imaging a trunk part, a body coil is used to transmit and/or receive RF pulse signals. FIG. 1 shows a patient bed 1, a gradient coil 2 and a body coil having a supporting tube 3, a plurality of capacitors 4, a connecting copper sheet 5 and a longitudinal copper sheet 6; the gradient coil 2 generates a gradient magnetic field, a patient lies on the patient bed 1 positioned at the center of the body coil; the longitudinal copper sheet 6 and the connecting copper sheet 5 are bonded on the supporting tube 3, the plurality of capacitors 4 are connected in series by the connecting copper sheet 5, and the connection of said connecting copper sheet 5 is broken at the positions of the capacitors 4, with the longitudinal copper sheet 6 connected to the capacitors 4 by the connecting copper sheet 5, so that the supporting tube 3, the capacitors 4, the connecting copper sheet 5 and the longitudinal copper sheet 6 form the body coil, which is mainly used for generating an RF field. In a prior art MRI system, a human body absorbs a part of the RF power, which is measured by a special absorption rate (SAR). If the transmitting power of the RF coil is relatively large, after having scanned for a period of time, the local surface temperature on the human body will rise under the effects of an electric field. According to U=I*Z, wherein U is the voltage of capacitors, I is the current flowing through the capacitors, Z is the impedance of the capacitors, and with I as a constant, the larger the impedance Z and the larger the voltage, then the higher the intensity of the RF field generated between the human body and the capacitors, the larger is the current induced on the skin of the human body, and the larger is the SAR; and the smaller the impedance and the smaller the voltage, then the lower the intensity of the RF field generated between the human body and the capacitors, the smaller is the current induced on the skin of the human body and the smaller is the SAR. Therefore, the impedance Z of the capacitors 4 affects the intensity of the RF field in its vicinity, which in turn affects the current induced on the human body and the human body's SAR. The capacitor's value C is in an inverse proportion to its impedance Z, therefore, if the value of the capacitor is large and the impedance small, the current of the human body generated under the intensity of the RF field is small; and if the value of the capacitor is small, the impedance is large and the intensity of the RF field nearby is large, then the induced current on the human body generated under the intensity of the RF field is large. The SAR of human body increases with the increasing intensity of the RF field on the surface skin, and decreases with the decreasing intensity of the RF field on the surface skin. Under normal circumstances, a patient is positioned in the middle of the supporting tube 3, so the shoulder and the abdomen parts of the patient are closer to the capacitors 4, therefore the SARs at the shoulder and abdomen parts are relatively high, and the local temperature at these parts may be increased. Some patients feel nervous or are concerned at such local temperature rise. In some prior art MRI devices, attempts have been made to avoid any significant temperature rise by way of increasing the capacitance and reducing the impedance. However, changes to the capacitance can lead to the changes in coil parameters, making the adjustments rather complicated.